1. Field of the Invention
The present invention relates to an image forming apparatus including a developing device such as a copying machine, a printer, a facsimile machine, or other similar image forming apparatus in which a premixed developer, i.e., a two-component developer including a mixture of toner and carrier is supplied to the developing device.
2. Discussion of the Background
There are, for example, two methods of supplying toner to a developing device in an image forming apparatus using a two-component developer for development: (1) a method of supplying toner to a developing device to make up consumed toner for development, and (2) a method of supplying toner and carrier to a developing device even though only toner is consumed for development. In the above method (1) of supplying toner, carrier particles get deteriorated by friction generated between the carrier particles while being agitated with toner in the developing device, resulting in carrier particles being crushed, and breakage of a coating film on the surface of the carrier particles. In addition, a charging amount of toner is increased by the friction between the carrier particles, so that the toner is not easily adhered to an image portion. As a result, the developing ability of the developer greatly decreases.
On the other hand, in the above method (2) of supplying toner and carrier, an excess developer in a developing device accommodating an excessive amount of developer due to the supply of toner and carrier is discharged from the developing device. The deteriorated developer in the developing device can be replaced with a fresh developer by repeating the developer supply and discharging operations. Therefore, the deterioration of carrier can be suppressed.
With regard to a background technique for suppressing the deterioration of carrier in a two-component developer, Japanese Laid-open patent publication No. 51-13249 describes an electrophotographic developing device including a detecting device that detects a ratio of carrier and toner in a developer in a developing section, a carrier supplying device that supplies a predetermined amount of carrier to the developing section based on a predetermined developing time or a predetermined copy volume, and a toner supplying device that supplies toner to the developing section. The toner is supplied to the developing section based on a detection result of the detecting device, and at least a predetermined amount of developer is discharged from the developing section.
Further, Japanese Laid-open patent publication No. 4-353881 describes a developing device including a housing that accommodates a two-component developer to be supplied to a developer carrier, a discharging device that discharges a premixed additional two-component developer into the housing, and a detecting device that detects an amount of the developer in the housing. The housing includes an exit port from which the developer is expelled. When the detection value detected by the detecting device equals a predetermined value or more, the amount of the developer in the housing is reduced by expelling the developer from the exit port.
According to the above-described two background techniques, an excess developer in a developing device is adequately discharged therefrom. As a result, the decrease of developing ability of the developer caused by the deterioration of carrier can be prevented.
In the above-described two background techniques, both toner and carrier are supplied to the developing section. Specifically, in the developing device described in JP publication No. 4-353881, a premixed developer including a mixture of toner and carrier is supplied into the housing. In the electrophotographic developing device described in JP publication No. 51-13249, toner and carrier are independently supplied to the developing section without being premixed. Therefore, the constructions of the developer supplying devices that supply a developer into a housing or a developing section differ between the developing device described in JP publication No. 4-353881 and the electrophotographic developing device described in JP publication No. 51-13249.
Specifically, the developing device described in JP publication No. 4-353881 includes only one developer supplying device that supplies the developer into the housing. The electrophotographic developing device described in JP publication No. 51-13249 includes two developer supplying devices, such as, a carrier supplying device and a toner supplying device. Thus, in the electrophotographic developing device described in JP publication No. 51-13249, the construction of the developing device becomes complicated, and the size and cost of the developing device increases as compared to the developing device using a premixed developer. Therefore, an image forming apparatus using a premixed developer supplied to a developing device has an advantage in practicality.
In an image forming apparatus, a residual toner remaining on a surface of an image carrier after a toner image is transferred from the image carrier to a transfer material has to be removed from the surface of the image carrier as a waste toner prior to a subsequent image forming operation. Such a waste toner needs to be discharged from the main body of the image forming apparatus.
With regard to a background technique for removing residual toner on an image carrier, Japanese Laid-open patent publication No. 63-298370 describes a toner cartridge including a volume ratio changing device which changes each volume ratio of a toner storing section and a toner collecting section. Specifically, after supplying toner to a developing device, the volume of the toner collecting section is changed to be greater than that of the toner storing section. The toner collecting section is used for accommodating waste toner. This construction allows an excess space in the toner storing section increased by supplying toner in the toner storing section to the developing device to be diverted to the waste toner collecting section. Therefore, as compared to a developing device in which each predetermined volume is secured for a toner storing section and a toner collecting section, the entire volume of the developing device does not increase, and a mixture of waste toner and fresh toner can be surely prevented.
In the image forming apparatus using a premixed developer to be supplied to a developing device, not only a residual toner removed from an image carrier but also an excess developer in a developing unit needs to be disposed of. In this case, because operations for discharging both the residual toner and excess developer from the main body of the image forming apparatus are necessary, the operation of the image forming apparatus is frequently stopped due to the disposal operation for the residual toner and excess developer, thereby causing downtime of the image forming apparatus. As the downtime of the image forming apparatus increases, the reliability of the apparatus decreases.
In the above-described developing device described in JP Publication No. 4-353881, it is necessary to provide a discharging device for discharging the developer and an excess developer container for containing the discharged excess developer. In addition, the image forming apparatus using the above developing device needs to include a waste toner container that contains the waste toner removed from the surface of an image carrier. When discharging the excess developer and waste toner from the excess developer container and waste toner container, respectively, the disposal operations need to be performed independently. Therefore, an operation time for discharging the excess developer and waste toner from the main body of the image forming apparatus is necessary, resulting in downtime of the image forming apparatus.
For example, Japanese Laid-open patent publication Nos. 9-218575 and 9-244376 describe developing devices for use in an image forming apparatuses in which an excess developer is efficiently discharged from the main body of the image forming apparatus to be disposed of. The above-described developing devices are configured so that a developer cartridge includes respective containing sections for an additional supply developer and an excess developer. In this configuration, when the developer cartridge is replaced with a new developer cartridge after all the additional supply developer in the containing section is consumed, the disposal of the excess developer is performed at the same time. In this case, an exclusive operation time for the disposal of the excess developer is eliminated, thereby preventing the downtime of an image forming apparatus caused by the disposal of the excess developer.
However, the image forming apparatus including the above developing device needs to include a waste toner container. Therefore, an exclusive operation time for the disposal of the waste toner is necessary, thereby causing the downtime of the image forming apparatus.
Further, Japanese Laid-open patent publication No. 11-212361 describes an image forming apparatus including a developing device in which an excess developer and waste toner are collected and stored in one collecting chamber. The collecting chamber is configured to be detachably attached to the main body of the image forming apparatus. In this image forming apparatus, the excess developer and waste toner can be disposed of at the same time by detaching the collecting chamber from the main body of the image forming apparatus.
However, the above-described image forming apparatus needs to perform a sequential operation for detaching the collecting chamber from the main body of the image forming apparatus, disposing of the excess developer and waste toner in the collecting chamber, and attaching the detached collecting chamber to the main body of the image forming apparatus.
Such a sequential operation is performed independently from a supplying operation of a fresh developer to the image forming apparatus, thereby causing downtime of the image forming apparatus.